The present invention relates to processes and compositions for reducing dioxygen. More particularly, the present invention relates to a catalytic process using dirhodium complexes to reduce dioxygen.
Extensive efforts have gone into finding efficient catalysts that can promote the reaction of dioxygen with oxidizable substrates. Biological catalysts are effective in activation and transport of dioxygen. Accordingly, substantial work has been done with metalloporphyrin catalysts. However, there are few additional examples of transition-metal complexes that have the desired properties and reactivity to be utilized in the catalytic activation of dioxygen.
Activation of molecular oxygen by transition-metal complexes can occur by way of a one-electron-transfer process, a two-electron-transfer process, or a four-electron-transfer process which generate the superoxide ion, peroxide ion, or oxide ion in aprotic media. The superoxide ion, O.sub.2.sup.-, is reactive towards some organic substrates either by nucleophilic substitution or by direct oxidation. Superoxide ions have been used for the degradation of chloroalkanes, chloroalkenes, and polychloro aromatic hydrocarbons. Solutions containing O.sub.2.sup.- can be prepared in aprotic solvents by electrochemical methods or by solublizing KO.sub.2 with 18-crown-6-ether. However, there is still a need to find convenient and inexpensive pathways to generate O.sub.2.sup.-.
A catalytic cycle involving manganese porphyrins for the reduction of dioxygen has been reported by S. E. Creager and R. W. Murray, "Electrochemical Reactivity of Manganese (II) Porphyrins. Effect of Dioxygen, Benzoic Anhydride, and Axial Ligands," 26 Inorg. Chem. 2612, (1987). Mixtures of the porphyrin and benzoic anhydride lead to a reduction of dioxygen at a control potential of -0.40 V vs SCE. This process involved reduction by more than two electrons and heterolysis of the O--O bond by benzoic anhydride. However, the manganese porphyrin complex did not show catalytic behavior in the absence of benzoic anhydride.
Accordingly, it would be a significant advancement in the art to provide a catalyst and process for the continuous generation of superoxide ion. Such a catalyst and process are disclosed and claimed herein.